1. Cross Reference to Related Application
Thermal ink jet resistors and direct writing thermal print heads have conventionally been fabricated by means of standard thick and thin film resistor deposition techniques. In one example of this technique as shown in FIG. 1 a thin layer of resistor material 10, such as 500 angstroms of tantalum/aluminum alloy is deposited on an isolation layer 15 such as silicon dioxide overlaying a silicon substrate 20. The isolation layer 15 provides the necessary electrical and thermal insulation between the resistive layer 10 and the silicon substrate 20. A conductive layer 30 such as 1 micron of aluminum is deposited on top of the resistance layer 10, and the conductive layer 30 and resistance layer 10 are patterned forming a resistor 40 connected by conductors 50. Finally, a passivation wear layer 60, for example 2-3 microns of silicon dioxide or silicon carbide, is deposited over the entire structure. The resistor 40 is then used to heat the ink or thermal paper which is just above the passivation layer 60.
In such film resistor devices, failures often occur in regions where there is a step height change in the surface profile such as region 70 in FIG. 1, which result from patterning the resistance layer 10 and conductive layer 30. Stress in the passivation wear layer 60 is highest in the step regions 70, and the occurrence of pin-holes is greatest along these steps.
It is possible to reduce the stress and pin-holes in the passivation layer 60 by making the passivation layer 60 thicker, but this is usually undesirable since it increases the thermal isolation of the resistor 40 from the ink or paper, thereby reducing heat transfer from the resistor 40 to the ink or paper and causing higher resistor temperatures which can induce further failures.
2. Summary of the Invention
Height changes in the passivation wear layer between the film resistor and the ink in a thermal ink jet printer or the thermal paper in a direct writing print head can be eliminated by fabricating the device in reverse order as compared to conventional film resistors and then etching away the underlying substrate. The result is an inverse fabricated resistor with reduced failures due to stress or pin-holes in the passivation layer.
A passivation film such as 1-2 microns of silicon dioxide or silicon carbide is deposited directly on a first substrate such as silicon or glass to form a flat, smooth passivation wear layer. This is followed by deposition and subsequent patterning of resistive and conductive layers, for example made of 500 angstroms of tantalum/aluminum and 1 micron of aluminum respectively. A thermal isolation layer such as 2-3 microns of silicon dioxide is then deposited over the resistor and conductor pattern, followed by a thick layer (10-1000 microns) of a metal such as nickel or copper, which serves as both a heat sink and support layer. The thick metal layer may then be bonded to a support bearing substrate and the first substrate is removed for example by etching.
The result is a film resistor overlain with a uniform, thin passivation wear layer which can be used to produce localized heating as needed in a thermal ink jet printer or in a contact thermal printing head with increased reliability over the prior art.